Frequency selective transmission system



Dec. 27, 1927. 1,654,123

R.-V. 1 HARTLEY FREQUENCY SELECTIVE TRANSMISSION SYSTEM Filed Sept. 50,1924 4 Sheets-Sheet 1 Mmvran:

1 Afro/1W0 Dec. 27, 1927. 1,654,123

R. V. L. HARTLEY FREQUENCY SELECTIVE TRANSMISSION SYSTEM fi l! A.HARTLEY ATTORNEY Dec. 27,1927. v 1,654,123

R. y. L. HARTLEY FREQUENCY SELECTIVE TRANSMISSION SYSTEM Filed Sept. 50,1924 I 4 Sheets-Sheet a 5' /A/VEN7'0/?.'

fiALPH-M L. HARTLEY AIME/vb V2 mummlnmuummi Dec- 27 O X R. V. L. HARTLEYFREQUENCY SELECTIVE TRANSMISSION SYSTEM Fild Sept. 30, 1924 "4Sheets-Sheet 4 FIG. /2.

VI llllllllll II 52 5:; a 67 hrexz/m mm 1/1. flfl/f/gz Patented Dec. 27,1927.

UNITED STATES PATENT OFFlCE.

RALPH V. L. HARTLEY, 0F SOUTH-ORANGE, NEW JERSEY, ASSIGNOR TO WESTERNELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., CORPORATION OF NEWYORK.

FREQUENCY SELECTIVE TRANSMISSION SYSTEM,

Application filed September30,1924. Serial No. 740,724.

The present invention relates to selective wave transmission such as thetransmission and selection of waves of particular frequen cies fordesired purposes.

The present invention further relates to vibrating systems andparticularly to the generation, translation and control of mechanicalvibrations.

It is a particular object of the invention to effect desired frequencyselections by means partly or wholly mechanical.

A further object of the invention is to provide for any desired bandwidth of frequency selection by complex mechanical structures.

A further object of the invention is to provide a system of the typeindicated which will be simple in construction and ellicient inoperation.

llt is a special object of this invention to provide for selectingmechanical vibrations by mechanical wave filters and for converting thevibrations into electrical variations.

A further object is to provide a signal transmitting system capable ofselecting mechanical vibrations ofdilferent frequencies representingdifferent signals and converting them into electrical variations fortransmission to a. distance.

Electrical filters generally comprise a reiterative series of sectionswhich may be I uniform throughout the entire filter network but which inpractice are generally made up of different types to suit the particularfilter requirements. Various types of electrical filters'and the theoryand design formulae for them are disclosed in the followingpublications; U. S. patent to Campbell No. 1,227,113, May 22, 1917 andan article by Otto J. Zobel in the Bell System Techni cal Journal forJanuary 1923 Vol. II, No. 1, pages 1 to 16.

The ideal electrical filter would have lumped inductances and capacitiesfree from resistance. Similarly the ideal mechanical filter would haverigid masses andmassless springs connected by massless connections andmoving without friction. In practice these conditions can never, ofcourse, be

realized, but in the design of any filter an approach to theseconditions may be made.

A field in which the mechanical filter may be used to particularadvantage is in carrier wave signaling. The limitations as to the numberof channels in such systems are determined by the character of theselective means available. -What is desired is a filter which introducesa uniform loss for frequencies Within the range of a particular channeland a very large loss for frequencies outside that range. It is knownthat a mechanical vibrator has a very much smaller damping and energydissipation than the best electrical circuit of practical construction.The entire resonance curve of a tuning fork of one thousand cyclefrequency, for example, was found to lie in a frequency; interval of theorder of one cycle. This type of selective element realizes the desiredcondition of low damping Within the resonance-frequency interval andvery high loss for a frequency only one or two cycles distant. The factthat the resonance interval is so extremely narrow, however, makes avibrator of this character practically useless for signal transmissionsince the frequency components representing the signal are cut off.

According to the invention, a number of mechanical vibrators arecombined with appropriate coupling elements to form a filter in whichthe low damping which is characteristic of this type of vibrator issecured over a considerable frequency range and in which very largelosses will be introduced 'for frequencies only slightly outside thetrolled in accordance with signals to be sent.

A more complete understanding of the invention may be had from thefollowing detailed description in connection with the accompanying,drawing; in which Figs. 1 to 8 inclusive, show types of mechanicalfilters corresponding to the electrical filters shown schematicallyin-the respective Figs. 1 to 8 inclusive;

Figs. 9, 10 and 12 show in schematic form the application of mechanicalfilters to carrier signaling systems in accordance with the invention;and

Fig. 11 shows a physical embodiment of one form of mechanical filter. v

Referring now to the figures illustrative of the different types offilter, Fig. 1 represents the general case of electrical filter of thetransmission type as distinguished from the suppression type. That is,each section is comprised of series inductance L and capacity C inseries with each other and shunt inductance Z and capacity 0 in parallelwith each other. \Vhen both series and both shunt elements are presentin each section the filter transmits two distinct bands of frequenciesand greatly attenuates currents of all frequencies except those includedin the two bands. The two bands are coalescent or confluent if LC=Z0,and by omitting one of the series elements from each section or one ofthe shunt elements, or both a series and a shunt element, the filter maybe transformed into one of the types shown in the other Figures 2 to 5"inclusive, and may be made to have only a single transmission bandhaving two finite frequency limits or having as one limiting frequencyeither zero or infinity. Each of these filters is of the transmissiontype as distinguished from the suppression type which will be definedhereinafter. All this is disclosed in Campbell Patent 1,227,113 to whichreference has been made.

In Fig. 1 showing the mechanical analogue of the electrical filter ofFig. 1", the mass M corresponds to the inductance L, the spring Kcorresponds to the reciprocal of the capacity C, and the couplingelements m and 7c correspond to the elements Z and c. Vibrations to betransmitted are applied at 1 and the output is taken off from 2. Corresponding to current flow through L, the mass M takes up motion fromthe impressed vibrations, but continued movement of M in one directionis prevented by spring K which stores energy similarly to theaccumulation of a charge on condenser C.

In each of the a figures no attempt is made to represent the appearancewhich the corresponding filter might present in actual practice, for thereason that it is desired to ado ta convention for the figures that willfaci itate comparison with the electrical type by emphasizing thepositions of the quantities. For instance in Fig. 1*, the elements M, K,may represent a single member which possesses appreciable mass andelasticity such as a tuning fork or single reed. The fork or reed mayhave either evenly distributed mass or may be weighted so that its massis partly or principally concentrated at a point along its len th. Theproperty of elasticity is indicated %y the zigzag line. The spring orreed is firmly secured at its lower end and the vibrations applied at 1are in the direction perpendicular to the plane of the drawing.

Secured to the top of the reed or spring is a pin 10 which passesthrough a hole in the rod 11 so that rod 11 is free to rotate about pin10. Suitable collars may be provided on pin 10 to maintain the rod 11 atthe desired level. The hole in rod 11 may be elongated to permit pin 10to move to and fro in a vertical plane perpendicular to the paper andstill permit rod 11 to move about pin 10' as a pivot Without elongatingor compressing spring 76.

Secured to rod 11 is the coupling mass m (coupling mass shown by a smallsphere in each instance). .Mass m is connected to pin 10 by the couplingspring is which preferably has its end fixedrigidly to pin 10.

The ideal case will be approached by concentrating, as far as possible,all of the mass n the spheres and by having the pin 10, rod 11, andsprings l: and K as light (massless) as possible.

Just as elements M, K may be embodied as a tuning fork or reed, theelements 11, m, is, may be embodied in a reed having one end fixed topin 10 and the other end mounted in any suitable manner to allow endwiseslipping relative to pin 10 and bending about pin 10, and this reed mayhave a weight located at a point along its length or may havedistributed mass.

In any case where the mass is concentrated it will be obvious to providea weight which is slidable along the rod or reed.

When sudden movements occur in M, the

mass m tends to remain stationary and the rod 11 in being moved by pin10 exerts a moment about 11?. as a center d's orting 7c. The mass m iscapable of a translatory motion without having to rotate. This effectmay be secured by rotatably mounting m on a vertical pin (not shown)'secured to rod 11 or the equivalent reed. A motion is thus communicatedto pin 10 and to mass M. As 70 becomes distorted m is constrained tomove. With these explanations of the interaction between the elements,the complete operation will follow clearly from that given in connectionwith Fig. 1.

From the conventions employed in all of the figures, it will be apparenthow each of the other types is constructed and operates.

Figs. 6 to 8 show typical forms of supfilter that its attenuation may bemade veryhigh at a frequency just outslde of the range of freetransmission, and the cut ofl of the lter therefore be made extremelysharp.

The design data for any of the types of filter that have been given mayreadily be obtained by use of the formulae given in the Campbell patentabove referred to or in the article by Zobel. a I

As an example, suppose that from convenience or other considerations itis desired.

Ell

till

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to build a band pass filter of the type shown in Fig. 3 and that thisfilter is to have free transmission for all frequencies between 900andlOOO cycles per second, and high attenuation for all otherfrequencies and that the mechanical resistancewithin the transmissionband is to be dynes/cm./sec., that is, v

such that an alternating sinusoidal force of 30 dynes is required tomaintain sinusoidal motion of the moving element with maximum velocityof 1 cm. per second.

Referring to Fig. 3 and to page 42 of the Zobel article it is seen thatthe type of filter chosen for the example is that correspond ing toZobelstype V1,. The design formales for the electrical and for themechanical types are then as follows, remembering that stiffness is thereciprocal of capacity:

Electrical Mechanical trated at the point indicated in the drawing andthat the amplitude of vibration is small compared with the distance ofthe mass from the center of rotation; If the vibrations are such thatthe mass has appreciable angular displacement, M would need to beexpressed in terms of moment of inertia. If thls were the case thestiffness of 7c and K would need to be expressed as moments of force perunit angular displacement.

The structure shown in Fig. 11 illustrates by Way of example only, theappearance which the filter of Fig. 3 may have in actual practice. 1 Thereeds may be steel bars or the like firmly clamped at their lower endsbetween bed pieces 41' and 42. Instead of single reeds, tuning forks,diaphra ms or other vibratory members may be use Secured to the reeds 40at a suitable point along their length, preferably at their free ends,are the coupling springs 43. By reference to Fig. 3 it 1s seen that.these coupling springs are massless, and hence are as light. aspossible, consistent with their re quired stiflness. Reeds A0, however,have mass and are indicated as comparatively. heavy. The mass may bedistributed as in the case illustrated, or may be lumped in the form ofa weight (not shown) secured t the reed.

In operation the end reed 40 is set into vibration in any suitablemanner, such as by the driving magnet 44 connected in any suitablecircuit 45 carrying variation currents and the vibrations aretransmitted through the coupling springs 43 to the successive reeds 40.The vibrations to which the structure is selective are passed withuniform small attenuation to the output eleinent 46 while vibrations ofall other frequencies are damped out. The filter is shown terminating ina mechanical resistance 47 the nature of which will be described morefully in connection with Figs. 9 and 10.

Reference will now .be made to the applications, shown in Figs. 9 and 10of the mechanical filter to typical signaling systems.

In Fig. 9, the main carrier line ML terminates in three carriertelegraph transmitters T T and T and in three carrier tcle graphreceivers R R and R The translfll) III) mitters may be of any suitabletype for sending Waves of respective frequencies and keys or relays forcontrolling the respective waves in accordance With messages to be sent.The frequencies employed by these transmitters are assumed to lie in ahigher range than the frequenciesemployed by the otherwise simi lartransmitters at the distant terminal for actuating the receivers R R andR Ac-' The receiving filter LF feeds into an am plifier A which maycomprise any number of stages but shown for simplicity as comprisingonly a single tube of the well known audion type. The amplified receivedwaves representing all three messages actuate elec-- tromagnetic relay15 causing its armature 16 to vibrate about the centers 17 and 18. Thesearmature vibrations are applied to the three mechanical filters F F andF which serve to filter out and transmit selectively to the vlvspcctiy'ereceivers R R and R the parend section may be connected in series with athe input end section of another or other filters of the same type. Tobe suited to this manner of connection a filter should have an impedancebetween its input terminals of the same order as the impedance of theload to which it is connected for those frequencies which it is totransmit selectively and this impedance must be high compared with theimpedance which it has for other frequencies. These filters areterminated mid-shunt in order to give them the proper terminalimpedance.

That the filters F F and F are series connected to the armature 16 maybe seen from considering the corresponding electrical filter of Fig. 2.If this filter is assumed to have a mid-shunt termination it Will haveas its terminal element a shunt-connected parallel-inductance-ca-pacityof .2Z and g.

e One indicatlon of the tseries connection 18 that'the input endsections of all the filters have the same motion, corresponding to theseries current through theend sections of electrical filters soconnected.

In the case of filters F F and F vibrations' of the frequency at whichthe midshunt elements 19 and 20, for example, resonate produce motion oflarge amplitude in these elements. Vibration of other frequen-- ciesresults either in displacing the mass 19 or distorting the spring 20 orin both of these effects but does not produce as great moveinent in theend of spring 20 attached to spring 21 as do the currents of resonantfrequency. This discriminating action is, 0t course, carried out by allof the other'elemerits of each filter in such manner as to se- The pin25 in vibrating taps against the downwardly extending armof the lever'26will be recognized as the analogue of a detector and relay. The rapidtapping by the pin 25 raises weight 28 to open contact 29, and due tothe inertia of the lever and weight, the contact is not again closeduntil the tapping ceases, that is, until the termination of the waverepresenting the dot or dash impulse.

The damping device 22, 23, 24 aids in giving the filter the desiredresistance termmation.

The system of Fig. 10 is similar to that of Fig. 9 except that thefilters F F and F are of the type shown in Fig. 3, this type of bandfilter being adapted for parallel connection, that is, its inputterminals may be connected in parallel with the input terminals ofanother or other filters. A requirement of filters for parallelconnection is that each should have an impedance of the same order asthe impedance of the load for currents of the frequencies which thefilter is to transmit and this impedance must be low compared with theimpedance which it has for currents of other frequencies. These filtersin Fig. 10 are preferably terminated-mid-series.

Received currents energize relay 15 as in the case of Fig. 9 and causeactuation of its armature. This armature is secured to a lever systemthe fulcrums of which are at 33, 34, and 36. Attraction of the armature(motion toward the back of the paper) tends to move pin 33 backward inthe drawing, pin 34 forward, pin 35 forward, pin 36 backward. It isevident that the motion absorbed by any filter is proportional in someinverse manner to the motion absorbed by the others. For example, if atany instant only the particular vibrations to which F is selective arebeing received, pin 33 will take up the motion on account of the lowimpedance of filter F to vibrations of these frequencies, while pins 34,and 36 will remain practically stationary on account of the highimpedance offered to these vibrations by filters F and F If onlyvibrations are received to which filters F and F, are selective, pin 33remains practically stationary and pins 35 and 36 take up the motion,the link between 35 and 36 oscillating about an imaginary fulcrumsomewhere along its length between pins 35 and 36.

The detecting and receiving elements may be identical with those of Fig.9.

Fig. 12 of the drawin illustrates two carrier telegraph transmit ingchannels which lit . dicated at T, for converting the mechanicalvibrations into electrical variations for transmission over any desiredcircuit L which may be the usual multiplex telegraph line. An amplifierA including any desired number or arrangement of vacuum tubes may beincluded as desired.

The vibrators V and V are shown in the form of notched discs secured tothe shaft 51 which is driven from the motor 52. The

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carrier frequency for each channel is determined by the speed ofrotation of the shaft 51 and by the number of notches in the discs sothat by providingdifferent numbers of 54 serve to apply to the filtersF, and F the mechanical vibrations tors V, and V The filters F, and Fare of the type indicated in Figs. ,2 and 9. The constants of the filterF may, in manner hereinbefore described, be proportioned so that thisfilter produced by the vibratransmits vibrations of the frequencygenerated at V and also transmits a sufficiently wide band offrequencies on either or both sides of the carrier frequency. to enablesignaling to be carried out atthe desired speed.

The filter F will have its constants differently proportioned from thoseof the filter F, so as not to transmit vibrations of the samefrequencies as those transmitted by the filter F but to transmitselectively the vibrations generated at V and such other frequencies asare necessary in order to transmit the signals on the correspondingchannel.

The translating means T includes the bar 55 and the electromagnet 56.The bar 55 is adapted to be given rotational vibrations about the axis57, 57 by the output elements 58 and 59 of the respective filters F andF The bar 55, therefore, partakes of vibrations of all of thefrequencies selectively transmitted by the filters associated with it.In vibrating, the bar 55 causes the armature 6O of the eleetromagnet 56to vibrate near the pole of the magnet and to set up corresponding E. M.Ffs in the magnet winding. The

winding should lmve a permanent magnet core or, some other means forestablishing a normal'field. The E. M. F.s generated in the winding 56are applied to the input of the amplifier A and the amplified waves areimpressed on the line L.

The filters F and F are illustrated as having their terminal sectionsconnected to the bar 55 in series, as is indicated by the factthatthe-output end sections of both filters have the same motioncorresponding to the current through the end sections ofelectrlcalfilters so connected. In order to better adapt the terminalimpedance of the filters for this type of connection, they are shownterminated mid-shunt, that 1s, the terminal coupling mass and elasticityeach have double the values of those of the interme diate sections ofthe filters. filters may be connected with their terminal sections inparallel with each other and in common to the translating device, inwhich case the filters should be terminated midseries instead ofmid-shunt. A more convenient filter for this form of connection is thatof the type shown in Fig, 3 and the manner in which the end sectionswould be associated with the translating device is, in general, similarto thatemployed in Fig. 10. As pointed out above, the internal impedanceof the filters should, in either type of connection, be of the sameorder of magnitude as the impedance of the load into which they work; 3

With the arrangements as thus far described, when the motor 52 is setinto vibra tion at the proper constant speed, vibrations of respectivelydifferent frequency are transmitted from the vibrators V, and Vcontinuously through the filters F, and F to the bar 55. In order tocontrol these vibrations in accordance with telegraph signals to betransmitted, stop members 61 and 62 are provided for acting upon anelement of the respective filter. These stop members are normally biasedby the springs 63 and 64 so that they bear upon the central serieselement of the filter with sufiicient pressure to arrest the motionofthis element. So'long as either stop member 61 or 62 is solely underthe influence of the biasing springs'therefor, the central element ofthe corres onding filter is held stationary and no vibrations aretransmitted by it from the central element on through the filter to thebar 55. The transmission of the vibrations in each channel may,therefore, be controlled by a1 If desired, the

engagement with the filter element and permitting the vibrations to befreely transmitted through filter F to the bar 55. When key 67 is raisedand electromagnet de energized, stop member 61 is brought to bear uponthe filter element, causing it to stop vibrating and the correspondingvibrations are cut off from the bar 55. Stop member 62 may be similarlyc-ontr0lled by electromagnet 66 and key 68=to control thetransmission'of vibrations through filter F What is claimed is:

1. In combination, a source of mechanical vibrations, means .fortranslating said mechanical vibrations into electrical variations, meansto control the application of the mechanical vibrations to thetranslating means in accordance with signals, and a mechanical frequencyselective device interposed between said source and the translatingmeans for freely transmitting tothe translating means vibrations withinpredetermined frequency limits only.

2. A signal transmitter comprising a plurality of mechanical vibratorsof respectively different frequency, a mechanical filter associated witheach vibrator to receive from it and to transmit selectively vibrationsof its particular frequency, and 'a common means arranged to receive thevibrations transmitted by all of said filters and to translate thevibrations into electrical variations.

3. In combination, a-mechanical vibrator, a device arranged foractuation by vibrations from said vibrator, a mechanicalfrequency-selective device interposed between the vibrator and thefirst-mentioned device, and means for acting on the selective device tocontrol application of vibrations to the first-mentioned device.

4. In a transmitting system, a continuously operating vibrator, amechanical wave filter having an input section arranged to be actuatedby said vibrator, and means acting on the wave filter for controllingtransmission through it of mechanical vibrations.

5. In a carrier transmitting system, a mechanical element capable ofvibration, a plurality of vibrators of different frequencies forimparting vibrations to said element, and means for translating thevibrations of said element into electrical variations.

6. In a carrier transmitting system, a plurality of mechanicalvibrators, means to translate the vibrations into electrical waves fortransmission, and means fo restricting the vibrationsthat are translatedto a fre-.

predetermined frequency .for transmission, means for restricting thevibrations that are translated to a frequency band of predeterminedfrequency level and Width, and means for controlling said vibrations inaccordance with signals.

8. In combination a mechanical wave filter and means for controlling thetransmission characteristic of said filter to modulate the vibrationstransmitted through the filter in accordance with signals.

9. A plurality of mechanical wave filters for transmitting vibrations ofdistinct frequency. ranges, unitary means actuated in common by thevibrations transmitted by all of said filters, and means individual toeach filter for controlling the application of vibrations to saidunitarv means.

10. In a -arrier telegraph svstem, means to generate mechanicalvibrations, means to modulate the vibrations in accordance with atelegraph message, a mechanical filter to select the vibrations, andmeans to translate the modulated and selected vibrations into electricalwaves.

11. A wave transmission system comprising means for continuouslyenerating mechanical vibrations, means or controlling said vibrations inaccordance with signals, and means for converting the resultantvibrations into electrical waves.

12. A signal transmission system comprising-means for generatingcontinuous mechanical vibrations of a definite frequency, means forconverting said vibrations into electrical variations, means selectiveof vibrations of said frequency for applying said vibrations to saidconverting means, and means for controlling the applied vibrations inaccordance with signals.

13. A communication system including a continuous source of mechanicalvibrations, means for converting said vibrations into electricalvariations and means for selecting said vibrations and applying themperiodically in accordance with signals to said converting means.

14. In a carrier telegraph system means for constantly generatingmechanical vibrations, means to modulate said'vibrations in accordancewith telegraph messages, means for selecting said vibrations, means totranslate said selected modulated vibrations into electrical waves andmeans for transmitting the electrical waves to a distant point.

15. A multiplex communication system comprising sources of continuousmechanical vibrations each of said sources being adapted to generatevibrations at a different frequency, means for converting saidmechanical vibrations into electrical waves, mechanical filtersassociated with each of said sources, said filters being selective ofthe respective frequencies of said sources, means for controlling theselected mechanical vibrations in accordance with signals and means forapplying the resultant vibrations to said converting means.

16. A mechanical wave filter comprising a series of elongated vibratablemembers securely mounted at one end and free to vibrate at the otherend, couplingmembers associated with the free ends of the vibratablemembers for communicating vibrations from one vibratable member to thenext, the members of one kind possessing considerable ,mass, and themembers of the other kind being capable of considerable distortion andwhen distorted of forcibly tending to restore to 'undistortedcondition,the structure as a Whole transmit-ting with substantially uniformattenuation vibrations comprising a band of frequencies and highlyattenuating and approximately extinguishing vibrations of neighboringfrequencies lying outside of said range.

17. A'mechanical Wave filter comprising a plurality of tuned reedssecured at one end to a rigid support and free to vibrate independently,and coupling members associated with the free ends of the reeds forcommunicating vibrations from one reed to the next in the series, saidcoupling members offering mechanical impedance to vibrations dependentin amount upon the frequency of the vibrations, said reeds and couplingmembers cooperating to transmit" from the first through the system andto produce in the last reed of the series vibrations comprising abroader band of frequencies than is comprised in the range of freevibration of any reed of itself.

18. A mechanical wave filter comprising a series of reeds-mounted forindependent vibration and yielding means coupling the free ends of saidreeds to each other.

19. A mechanical Wave filter comprising a series of reeds mounted toleave free ends capable of independent vibration, yielding memberscoupling the free ends of said reeds to each other, means to impressvibrations .upon the first reed of the series, and a signaling elementactuated by another reed of the series.

20. A selective system comprising a plurality of mechanical filters eachcomposed of a series of vibratable members, and coupling members forcommunicating vibrations from one vibratable member to another in theseries, the elements of said .filters being differently proportioned inthe respective filters to determine a different range of frequencies forfreely transmitted vibrations for each filter, a common means forapplying to all of the filters vibrations of frequencies within theirselective transmission ranges and an individual vibration-responsi 3device operatively associated with an out ut element of each filter. I21. A selective system according to claim 20, said mechanical filtersbeing terminated adjacent said common means in fractionalsectiontermination.

- 22. A selective system for mechanicalvibrational energy comprising a'plurality of multi-section mechanical Wave filters each selective tovibrations of a different range of frequencies, a commonvibrationally-operating device connected to all of said filters inseries so that the element of each filter connected to said device hasthe same motion in the case of all the filters, the element connected tosaid device offering in the case of each filter high mechanicalimpedance to vi-- brations of the frequencies to which the respectivefilter is selective, and offering lower mechanical impedance tovibrations of other frequencies.

23. A selective system according to claim 22, each filter of which isconnected to said common device by a fractional-shunt section giving thefilter a terminal impedance of the same order of magnitude as theimpedance of said device.

24. A selective system for mechanical vibrational energycomprising aplurality of multi-section mechanical wave filters each selective tovibrations of a different range of frequencies, a commonvibrationally-operating device connected to said filters in par allel,said device being connected to an element of each filter so as to dividethe vibrational motion among the saidelements, depending upon thefrequency of the vibrations, the element connected to said deviceolfering in the .case of each filter high mechanical impedance tovibrations of all frequencies outside the range to which the re-Speotive filter is selective and offering lower mechanical impedance tovibrations of frequencies Within the selective range of the respectivefilter.

25. A selective system according to claim 24;, each filter of which isconnected to said common device by a fractional-series section givingthe filter aterminal impedance of the same order of magnitude astheimpedance of said device.

In Witness whereof, I hereunto subscribe my name this 23 day ofSeptember, A. D. 1924.

RALPH V. L. HARTLEY.

